Systems for analyzing three-dimensional seismic data collect seismic measurements from configurations of geophones that are attached to and communicate with digitizer units (DUs), which that produce signals that correspond to the seismic measurements. The DUs, which are placed at selected locations along cables that span a site of interest, take readings from the configurations of geophones and send corresponding signals over the cables to a data recording and control center. The data is used to determine, for example, the likelihood that oil reserves are present on the site. A typical site may include thousands of DUs, each with an associated configuration of geophones.
Setting up and operating the measurement system is both time consuming and complex. The locations of the DUs must be known to within tight vertical and horizontal accuracy limits, and generally, the locations of the respective DUs are individually determined using optical survey techniques, GPS RTK “back-pack” systems or GPS/INS systems. In addition, the sites can no longer be cleared, that is, foliage cannot be removed at the selected locations of the DUs, and thus, the process of determining the locations of the DUs is made even more time consuming and complex when lines of sight are blocked.
The geophone readings taken by the DUs must be synchronized to within tight timing limits. In prior known systems, timing signals are sent along the cables and readings are taken at the various DUs in response to the signals. Accordingly, the lengths of the respective cables are limited, to avoid associated timing signal delays. Consequently, the area that can be tested at a given time is also limited by the lengths of the cables.
What is needed is a system that can be set up in a less time consuming manner and operate accurately over larger areas.